Marine Caulk and Adhesive Sealants Explained

The majority of marine-specific sealants are one of three material types: Polysulfide; Polyurethane; and Silicone. A fourth kind of sealants are hybrid silicone and polyurethane compounds that provide benefits of both materials.

In addition, there are application-specific teak deck caulking product designed to fill the grooves between teak planks on boat decks. Lastly, there is a group of items associated with traditional wooden boatbuilding, and designed for caulking wood planking seams both above and below the waterline. Examples of classic wooden boat caulk items include Oakum, Caulking Wick, Caulking Cotton, Caulking Irons, Bedding Compounds, Seam Compounds, and Seam Cement.

Choosing the correct type of adhesive or sealant is easy once the application, materials to be bonded and the sealant characteristics are understood.

Polysulfide Caulk

Items and Brands: Boatlife Life Calk

Where to use: Polysulfide sealants are ideal for teak decks, bedding deck and hull hardware, sealing thru-hull fittings, and underwater seams. They bond to fiberglass, wood, metal, glass, and themselves.

Polysulfide caulk is a one-part synthetic rubber material that makes a very versatile marine sealant. This type of caulk will bond readily to fiberglass, wood, metals, glass, and to itself. Polysulfide cures to a firm flexible rubber seal with excellent waterproofing and adhesion qualities. It can be applied underwater for emergency repairs, and or on damp surfaces. Please note: The solvents contained in polysulfide sealants may chemically react with and degrade many plastics, and this why they are not to be used on ABS, PVC, acrylic and polycarbonate materials as noted above.

Polyurethane Adhesive Sealants

Where to use: Polyurethanes are ideal for thru-hull fastening, and best choice for hull-to-deck joints, deck fittings, railings and transducers. They bond tenaciously to fiberglass, wood, metal, and glass.

Do not use for: Bonding parts that may require disassembly in the future, as Polyurethanes are generally permanent. Do not use to bed plastic windshields, plastic portlights (acrylic aka Plexiglas; or polycarbonate aka Lexan).

Cure time: A week or less.

Waterline: Used above and below the waterline.

Chemical Resistance: Teak cleaners can degrade the sealant and compromise its strength.

Polyurethane adhesives form an extremely strong bond while retaining flexibility and strength above or below the waterline. Watertight bonding / sealing of fiberglass, gelcoat and wood. Alcohol should not be used when preparing surfaces for bonding as it will stop the curing process and cause the adhesive to fail. Polyurethane is not recommended for use as a teak deck seam sealer. Extended exposure to chemicals (teak cleaners, oxalic acid, gasoline, strong solvents and other harsh chemicals) may cause permanent softening.

Marine Silicone Sealants

Where to use: Head plumbing applications, interior & exterior wood, trim sealing and as a gasket adhesive.

Do not use for: Silicone Sealant is not recommended for the installation of glass, polycarbonate (Lexan) or acrylic (Plexiglas) windows that are not also mechanically fastened. Do not use silicone on any surface to be painted.

Teak deck caulks are typically one-part, neutral curing, adhesive sealants specially formulated for caulking teak decks with outstanding adhesion to teak and other naturally oily woods. When used as directed, no primer is necessary. The caulk cures when exposed to moisture in the air to form a tough, flexible, solid rubber seal with excellent temperature stability and resistance to chemicals, sunlight, UV and water. Teak deck caulks are non-corrosive to all substrates and can be sanded when cured.

Traditional wooden boats are constructed and maintained with these 'old time' products. The caulking on wooden vessels uses fibers of cotton, and oakum. These fibers are driven into the wedge-shaped seam between the planks with a caulking mallet and a broad chisel-like tool called a caulking iron. The caulking is then covered with a seam compound in the case of hull seams, or else in deck seams with melted pine pitch, in a process referred to as paying. Modern marine sealants are frequently used now in place of the pitch, or even to supplant the oakum and cotton.